The operation of contemporary devices for detecting electromagnetic signals rely, in a known manner, on two different principles:                Very broadband direct detection, the principle of which is illustrated by FIG. 1,        Narrowband superheterodyne reception, the principle of which is illustrated by FIG. 2.        Very broadband direct detection, in a band extending for example between 2 and 18 GHz, exhibits the advantage of hardware simplicity. Its cost of implementation is therefore relatively low. Moreover, it allows, in principle, instantaneous visibility over the whole spectral band of interest. Therefore the probability of intercepting a signal is advantageously a maximum. However, the major drawback of this principle is that it does not exhibit frequency selectivity, this being considered to be penalizing when operating in a dense environment, that is to say filled with radioelectric signals of diverse frequencies.        
An essential characteristic of superheterodyne reception is that it is selective on account of the use of a bandpass filter, defined as being appreciably narrower than the totality of the band to be covered (the filter of intermediate frequency: Fl). Accordingly, it entails transposing the signal into intermediate frequency.
Therefore, total coverage of the spectral band of interest, 2 to 18 GHz for example, is conventionally done by scanning the latter, but to the detriment of the probability of intercepting transitory signals such as pulses. To avoid this annoyance, it would be necessary to place in parallel as many reception chains of this type as there is Fl bandwidth in the total band to be covered. This would have the direct consequence of excessively increasing the hardware complexity of the reception function, and therefore its cost. Consequently, it is not generally done.
Therefore, the current tendency, when using a superheterodyne reception system, is to widen the Fl passband and, in order to preserve the desired selectivity, to undertake a spectral analysis in the Fl band, by FFT (Fast Fourier Transform) for example. However, the probability of interception remains lower than in the case of direct broadband reception. Moreover, this principle of narrowband reception, which limits the microwave-frequency noise band, improves sensitivity only to a modest extent. Furthermore, the proliferation of pathways of complete reception (superheterodyne receiver and spectral analyzer), to process in parallel the signals arising quite often from at least four antennas, has the consequence also of leading to hardware complexity and therefore a high cost.
These two types of reception are generally associated with antennal devices exhibiting little gain. The principle observed is generally to cover an angular span of 360° in bearing with a restricted number of antennas, so as notably to minimize the number of parallel reception pathways required to simultaneously process the signals corresponding to the intercepted wavefront. The number of antennas being restricted, use is made of antennas exhibiting low directivity and therefore a lesser gain. This low antenna gain is therefore prejudicial to sensitivity, on which it has a direct influence, in contradistinction to the reduction of microwave-frequency noise band.
For certain applications, it is also known to use systems for superheterodyne reception comprising rotating antennas with large gain. However, the probability of interception of such systems remains very low. Indeed, the effective coverage of the spectral band of interest and of the desired angular aperture, 360° in bearing for example, makes it necessary to undertake a sizable number of scans of the frequency-bearing space.
Hence, in a general manner, the receivers are not truly adapted to signal durations that may vary within a wide span, thus preventing them from being optimal in terms of sensitivity, in particular for waveforms with very low peak power, the waveforms implemented by emissions of LPI (Low Probability of Intercept) type, for example, waveforms with linear frequency modulation (i.e. FMCW or Frequency Modulated Continuous Wave), or else waveforms with phase code modulation.